1. Field of the Invention
This invention relates to polymer films and more particularly to multi-layer polymer films. The invention relates, in particular, to a multi-layer polymer film having a substrate layer of an oriented polyolefin film such as an oriented polypropylene film.
2. Description of Related Art
The packaging industry requires large quantities of heat-sealable film which can be sealed to itself at a temperature which will not impair the film integrity. Polyolefins are in demand as film forming materials owing to their strength and clarity but, in general, exhibit relatively poor heat sealing characteristics. This is especially true of polypropylene film.
Attempts to improve the heat sealing characteristics have generally involved the application of a heat sealable layer to one surface of a polyolefin film base. The heat sealable layer is composed of a material which can be melted or partially melted at a temperature below that at which the polyolefin begins to soften. To be effective, the heat sealable layer must be composed of a material which allows for sealing over a relatively wide temperature range and which adheres securely to the polyolefin film base. Furthermore, the heat sealable layer must be able to provide a bond of adequate strength when it bonds to itself. This latter characteristic is usually termed the heat seal strength.
Heat sealable layers or coatings comprising a copolymer of ethylene and an unsaturated carboxylic acid, vinyl acetate or vinyl alcohol are well known in the packaging industry. One layer which has found particular favour comprises a copolymer of ethylene and acrylic acid. These materials have certain advantages and, in particular, can be sealed at relatively low temperatures to provide a water resistant seal which is resistant to crazing when flexed. However, the nature of conventionally used heat sealable materials is generally such that the resulting film tends to exhibit the phenomenon of xe2x80x9cblockingxe2x80x9d in which unwanted adhesion occurs between two layers of film which are placed in contact under moderate pressure as may occur during processing, use or storage of the film. Films exhibiting this problem do not perform well on film handling equipment and attempts to peel apart adjacent layers of blocked film may severely damage the film surface. As a result, heat sealable layers tend to be applied at the time of use to avoid the problem of blocking.
The susceptibility of a film to blocking may be reduced by incorporating a particulate additive, such as silica and/or alumina having a mean particle size of up to about 20 xcexcm, into the heat sealable layer. However, the incorporation of such particulates into the heat sealable layer will often detract from other desirable film characteristics, such as optical clarity, colour and printability. Moreover, the presence of particulate additives may generate an unacceptable accumulation of abrasive debris on, and around, the film, handling equipment. In general, measures adopted to inhibit blocking usually adversely affect the heat sealing characteristics of the film, particularly the sealing range and threshold temperature.
WO-94/25270 describes a base polymer film coated with a heat sealable layer comprising a copolymer of 65 to 95 weight % ethylene and 5 to 35 weight % (meth)acrylic acid in which 2 to 80% of the carboxylate groups are neutralized with metal ions from Groups Ia, IIa, or IIb of the Periodic Table. The resulting film is said to have improved hot tack and blocking properties without an unacceptable sacrifice of low minimum seal temperature. In particular Table II reports minimum seal temperatures (to reach 100 g/25 mm seal strength) of above 89xc2x0 C. The seal which formed is also claimed to be resistant to water immersion. Nevertheless, the presence of the metal ions, which are non-volatile and thus remain in the layer, tends to confer undesirable moisture sensitivity which can result in blocking of the film in moist or humid environments or the loss of seal strength when packing moist products. In addition, the hot tack performance of the seal, i.e. the strength of the seal when heated to a temperature at or above the melting point of the polymer forming the seal, tends to be unsatisfactory.
EP-A-229,910 discloses a heat-sealable polyolefin film coated on either one or both sides with a composition comprising 40 to 70% by weight of an acrylic polymer, 15 to 30% by weight of an alkyl(meth)acrylate/styrene/unsaturated carboxylic acid copolymer and 15 to 30% by weight of a vinyl ester/unsaturated amide copolymer. The sealing temperature reported in Table I is 130xc2x0 C.
Therefore the goal of this invention is to achieve a multi-layer film having a heat-sealable layer avoiding the various problemsxe2x80x94blocking phenomenon, moisture sensitivity, insufficient tack performance of the sealxe2x80x94of the prior art and having a low heat-sealing temperature.
We have now developed a multi-layer film having a heat sealable layer which can ameliorate or substantially eliminate one or more of the aforementioned problem
According to the present invention there is provided a multi-layer film comprising:
a.) a substrate layer of a thermoplastic polymer film having opposed first and second surfaces; and
b.) a heat sealable layer associated with the first surface of the substrate and comprising
(i) a first acrylic polymer and
(ii) a second polymer which is a copolymer comprising residues derived from one or more olefinically unsaturated monomers and residues derived from one or more olefinically unsaturated monomers which comprise one or more functional groups,
characterized in that the first polymer (i) is an amorphous polymer having a glass transition temperature in the range of from 40 to 120xc2x0 C. and a weight average molecular weight (Mw) in the range of from 20,000 to 60,000, the second polymer (ii) is a copolymer of a C2-C4 olefinically unsaturated hydrocarbon and an olefinically unsaturated carboxylic acid, and the heat-sealable layer comprises from 5 to 60% by weight of the first acrylic polymer and from 40 to 95% by weight of the second polymer.
The multi-layer film is a self-supporting structure in the sense that it is capable of independent existence in the absence of a supporting base. The heat sealable layer is generally coextensive with the substrate layer.
The present invention also provides packaging material comprising the multi layer film defined above.
The heat sealable layer of the multi-layer film of the present invention will preferably have a heat sealing temperature in the range of from 50 to 90xc2x0 C., more preferably in the range of from 60 to 80xc2x0 C.